"paper birch." The Columbia Encyclopedia, Sixth Edition. 2008. Encyclopedia.com. 24 Dec. 2009 <http://www.encyclopedia.com>.
"paper birch." The Columbia Encyclopedia, Sixth Edition. 2008. Encyclopedia.com. (December 24, 2009). http://www.encyclopedia.com/doc/1E1-X-paperbir.html
"paper birch." The Columbia Encyclopedia, Sixth Edition. 2008. Retrieved December 24, 2009 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-X-paperbir.html